CHAPTEE XIII. 



THE ANIMAL CELL. 



THE cell constitutes the morphological unit of all animal and 

 vegetable life, and as such is capable of manifesting those peculiar 

 activities which we regard as characteristic of living matter. In 

 its simplest form it represents a tiny bit of a more or less granular, 

 gelatinous substance the so-called protoplasm in the interior of 

 which a somewhat more solid-looking body can be made out, which 

 is termed the nucleus. Such simple cells exist in nature, either as 

 such or as conglomerations of many cells which represent the higher 

 forms of animal and vegetable life. All living matter, however, 

 whether simple or complex, has for its origin the single cell. But 

 while in the lowest forms of life the single cell is capable of per- 

 forming all those functions which are characteristic of living matter 

 by itself, we find, as we ascend in the scale of animal and vegetable 

 life, that certain groups of cells are here set aside for the purpose 

 of executing separate functions. Such groups of cells we speak of 

 as tissues, and we accordingly find in the highly organized mammal 

 a differentiation of the entire body into tissues, which according 

 to their functions may be grouped as tissues of locomotion, of re- 

 production, of digestion, of excretion, etc. With such a differentia- 

 tion of cells into tissues, however, the original aspect of the cell is 

 more or less changed. The highly diiferentiated voluntary muscle- 

 cell would thus at first sight scarcely be recognized as being in any 

 way related to the oval cell from which it is primarily derived. 

 On careful examination, however, we find that, no matter how 

 unlike its ancestral cell such a specialized cell may appear, the dif- 

 ference is only apparent, for all cells of the body consist at one 

 time of their existence at least of protoplasm and nucleus. The 

 striated portion of the muscle-cell is thus nothing more than the 

 protoplasm of the original cell, diiferentiated and modified in accord- 

 ance with the function which the cell is to perform. In some cells, 

 however, such as those of the adipose tissue, the original differentia- 

 tion into protoplasm and nucleus is apparently lost, and on ordinary 

 microscopical examination it appears that such cells are nothing but 

 large globules of fat. But with special methods of staining we can 

 demonstrate even here that there are a nucleus and protoplasm. The 

 only cells, in fact, in which a nucleus cannot always be demonstrated 

 are the red corpuscles of the circulating blood of man and the 

 anthropoid apes. \Ve find, however, that even in adult man all 



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